Encoders for position measurement are required in many fields of application, in which the position of an element movable along a path, for example, the position of a machine component on a linear axis such as drives or pivot arms, is to be determined. The positions detected in this case can be used as position values for measuring purposes, or also for positioning components by way of a drive having a position control loop. Such position encoders are accordingly used in devices such as coordinate measuring machines (CMM), geodetic devices, robot arms, elevators, train systems, radar, sonar, communications, acoustics, optics or hydraulic actuators.
An encoder has for this purpose a scale and a read head for the scanning thereof, which are movable in relation to one another, as well as a processor for regulating measurement operations and for assigning a position value, which is recorded by the read head, to a scanning signal. Depending on the requirements and structural options, either the read head is stationary and the scaler is movable, or the scaler is fixedly positioned and the read head is moved in relation thereto.
The scanning of the scaler is performed in a contactless manner and generally based on optical, inductive, magnetic, or capacitive physical principles. For example, the read head can have a light source, which irradiate a scaler having light-reflective or light-scattering (reflected light scanning) or light-transmitting (transmitted light scanning) markings with light. The light is incident therefrom on a light-sensitive pickup of a sensor, for example, on a photocell or a CCD array. If the scaler has light-reflective markings, the sensor is attached on the same side of the read head as the illuminating means. In the other case, the read head laterally encloses the scaler and sensor and illuminating means are arranged approximately opposite to one another in/at/on the read head. The markings are used as code elements for coding the relative position of the read head in relation to the scaler. The coding can be implemented as an incremental code, for example, by alternating similar light/dark transitions, moiré patterns, or, for measurement methods other than optical measurement methods, as an electrical or magnetic poles, or as an absolute code, for example, by a number of defined different patterns. See, e.g., U.S. 2015/0233742.
The accuracy of the position depends on resolution of the scaler, frequency of the transmitted signal, and sensibility of the encoder to the scattering events caused by dust and dirt in industrial environment. Accordingly, there is a need to improve the accuracy of estimation of relative position of the encoder suitable for industrial applications in dusty and smoky environment.